How is the dose of chlorine calculated?

The dose of chlorine is calculated by taking into account both the chlorine demand and the desired residual. Chlorine demand refers to the amount of chlorine that is consumed by various reactions in the water, including oxidation of organic materials, reactions with ammonia, and other contaminants. The residual is the portion of chlorine that remains in the water after these reactions have taken place, which is essential for maintaining disinfection as the water moves through the distribution system.

To ensure effective disinfection, it's crucial to provide a sufficient amount of chlorine that meets the demand while also achieving the target residual concentration. The formula of combining both the demand and the desired residual provides the total chlorine dose needed for treatment. This method ensures that there is enough chlorine to handle the contaminants while still leaving an adequate concentration of chlorine that can continue to protect the water from microbial growth during distribution.

Other methods of calculating the chlorine dose, such as considering residual alone or demand alone, would not provide the complete picture necessary for effective treatment. For instance, simply using residual without accounting for demand could lead to insufficient disinfection if the chlorine is not meeting the actual needs of the water being treated. This makes the sum of demand and residual the most comprehensive and effective way to calculate chlorine dosage.